1. Field of the Invention
The present invention relates to a color image forming apparatus for forming a multicolor image by forming an image of each color, and more particularly to an apparatus with a controller capable of processing a plurality of sheets for each color.
2. Related Background Art
Conventionally, an apparatus has been known wherein a plurality of color images developed by developing units for respective colors are superimposed on a transfer paper carried on a transfer drum onto form a full-color image, with a color image forming method referred to as a multiple transfer method, using the electrophotographic technology, for example.
Also, such an apparatus has been known wherein color images are sequentially superimposed on a plurality of transfer members carried at different positions on a transfer drum so as to improve the full-color image forming speed per unit time.
However, for controlling the retention of a plurality of transfer papers as in the above-described conventional example, there are a number of considerations such as the original reading length and the optical system driving motor speed, the developing unit travel distance and the developing unit driving motor speed, etc., so that it is impossible to determine whether the retention of a plurality of sheets is possible or not by taking into consideration only the size of transfer paper.
FIG. 3 shows an instance in which two A4-sized transfer papers are retained on a transfer drum to copy an original D in the above example. When the peripheral length of the transfer drum is e.g. 502.4 mm and two A4-sized papers are retained, the transfer paper interval is 41.2 mm, or equivalently about 257 msec at a predetermined rotation speed of the transfer drum (FIG. 3A). The read scanning of the original is performing in such a way as to start to accelerate the optical system driving motor from 1 a distance S forward of the original D, reaching a predetermined speed Vfmm/sec by 2 at a leading edge of the original D, scanning at a constant speed a distance L to 3 at a trailing edge of the original, then decelerating the motor, and stopping at a position 4 a distance S backward of the original D. Thereafter, for the next read scanning operation, the system is moved back to a position 1 at a maximum speed of Vbmm/sec (FIG. 3B). That behavior is shown in FIG. 18, in which the speed V is indicated at the vertical line and the time t at the transversal line. For the copy of two retained papers, the time period from the time t1 at the completion of image reading from the first transfer paper to the time t2 at the start of image reading from the second transfer paper, i.e., (t2-t1), must be equal to or less than the above-mentioned transfer paper interval time, i.e., 257 msec. Accordingly, the possibility of the retention of two sheets must be determined depending on the reading speed or copying magnification which is a deterministic factor for the rising or falling time ts, as well as the scanning distance L which is a deterministic factor for the movement back time tb. Further, on the two-sheet retention print, it is necessary to consider the time required to move the developing unit. For example, when the developing units for four colors of Bk, Y, C and M are arranged on a line, the developing units are positioned in four ways with respect to a photosensitive drum, as shown in FIG. 4A. The travel time from one developing unit position to the other is shown in FIG. 4B. The vertical line indicates the position before movement and the transversal line indicates the position after movement. The time required for the movement between neighboring developing units (M-C, C-Y, C-Bk) is td1, the time required for the movement between one-position skipped developing units (M-Y, C-Bk) is td2, and the time required for the movement of M-Bk is td3. For the print of two retenting sheets, the movement between used developing units must be performed within the previously-mentioned transfer paper interval time, i.e., 257 msec. For example, the possibilities of td1&lt;257 msec&lt;td2&lt;td3 are shown in FIG. 4C. For example, when two developing units are used, blue and green are possible (C) but red is impossible (x). Further, there is a case, such as the four-color, that the retention of two sheets is possible, but a margin equivalent to one sheet must be taken between the print of first two sheets and that of next two sheets for the return from Bk to M (.DELTA.).
There was a problem that it was difficult to make an appropriate control to cope with such different occasions, or became more complex even if possible.